Imaging of Airspace Lung Disease

1

What is the difference between a pulmonary acinus and a secondary pulmonary lobule?

The acinus (Latin for “berry”) is a structural unit of the lung distal to a terminal bronchiole, supplied by first-order respiratory bronchioles, which contains alveolar ducts and alveoli. It is 0.6 to 1 cm in size and is the largest unit in which all airways participate in gas exchange.

The secondary pulmonary lobule contains up to 25 acini, is 1 to 2.5 cm in size, is polyhedral in shape, and is the smallest unit of the lung that is surrounded by connective tissue septa. Its centrilobular, or core, structures include bronchioles, pulmonary arterioles, and lymphatic vessels, whereas the peripheral interlobular septa contain pulmonary veins and lymphatic vessels.

2

When a patient is suspected of having airspace lung disease, what is the first imaging method of evaluation that is utilized?

Chest radiography. Although the sensitivity and specificity are not as high as in chest computed tomography (CT), the cost and radiation dose are significantly less, and the results are often diagnostic.

3

When is chest CT used to assess airspace lung disease?

Chest CT for assessment of airspace lung disease is usually not necessary because common causes (e.g., pulmonary edema and pneumonia) will resolve with appropriate therapy. Chest CT is utilized when airspace disease persists despite intervention or when a patient's clinical picture does not align with the radiographic imaging appearance.

4

True or false: Referring to an “opacity” on chest radiography or CT implies airspace disease.

False. The term opacity is a nonspecific descriptor that implies a region that attenuates x-rays to a greater degree than surrounding tissues. This can be due to any abnormality overlying or within the lung such as skin thickening, a mass, a pleural effusion, or airspace disease.

5

What is the definition of consolidation?

Consolidation specifically refers to an alveolar filling process, which renders the lung opacified ( Figure 18-1 ). This appears as a homogeneous increase in lung attenuation that obscures vessels and airway walls, and may be focal, multifocal, or diffuse in distribution.

6

What is the “silhouette” sign?

The majority of structures visible on chest radiographs are due to the attenuation difference with air in the adjacent lung. Thus, the heart and mediastinal borders, the borders of the chest wall, and the vessels within the lungs are all seen because they attenuate x-rays to a greater extent than the adjacent air-filled lung. If the lung adjacent to one of these structures is consolidated, then there is no longer an attenuation difference between the lung and the structure, and so the structure and the normally visualized border disappear. This phenomenon has been termed the “silhouette” sign and is a specific marker of airspace lung disease (see Figure 18-1 ).

This sign is useful to help localize sites of lung disease. For instance, if the diaphragm is obscured, this implies lower lobe lung involvement. If the right heart border is obscured, this denotes a right middle lobe lung involvement. If the left heart border is obscured, this implies left upper lobe (lingular) lung involvement. If the aortic knob or right paratracheal stripe is obscured, this implies upper lobe lung involvement.

7

What is an air bronchogram?

In a normal chest radiograph, the air-filled bronchi are not distinguishable from the surrounding air-filled alveoli. If the alveoli become consolidated, then there is an attenuation difference between the air-filled bronchi and the surrounding fluid-filled alveoli. As a result, one or more bronchi become visible as dark branching lines within the opacified lung. This phenomenon is termed an “air bronchogram.” The presence of an air bronchogram is a specific marker of airspace lung disease (see Figure 18-1 ).

8

What is the difference between an acinar opacity and an air alveologram?

An acinar opacity refers to an ill-defined nodular opacity in the lung measuring 0.5 to 1 cm due to opacification of an acinus with surrounding aerated lung. Acinar opacities may coalesce and lead to larger areas of consolidation.

An air alveologram is the converse and refers to a small rounded lucency in the lung when a normally aerated acinus is surrounded by opacified lung parenchyma. Both are signs of airspace lung disease.

9

What is the definition of ground glass opacity?

The term ground glass opacity was first described on CT of the chest and refers to an increase in lung attenuation that does not obscure the pulmonary vasculature ( Figure 18-2 ). This is in contrast to consolidation which obscures underlying bronchovascular anatomy and appears much more opaque. Like consolidation, ground glass opacity can be focal, multifocal, or diffuse and can be seen in acute or chronic conditions. It can be caused by partial filling of airspaces, interstitial thickening, decreased lung aeration, and/or an increase in capillary blood volume. There is no direct correlate of ground glass opacity on chest radiographs. Paradoxically, some patients with ground glass opacity on CT examination will have chest radiographs that appear normal, whereas others will appear as consolidation and yet others will appear as a faint increased opacification of the lungs.

10

What is the crazy paving pattern?

This is a CT-related term that refers to presence of pulmonary ground glass opacity with superimposed interlobular septal thickening and intralobular lines, resembling irregularly shaped paving stones ( Figure 18-3 ). The crazy paving pattern was originally associated with pulmonary alveolar proteinosis (PAP), but it can be seen in a wide variety of disease conditions including infection, hemorrhage, pulmonary edema, and some chronic disorders.

11

What are centrilobular nodules and tree-in-bud opacities?

Centrilobular nodules are small (less than 1 cm) focal opacities located in the centers of secondary pulmonary lobules on CT, which spare pleural and fissural surfaces and are often ill-defined. These are due to pathologies that affect the bronchioles, pulmonary arterioles, and lymphatic vessels found in the central portions of secondary pulmonary lobules. Although seen in various disease conditions, their presence usually indicates infectious or inflammatory small airways disease such as by hypersensitivity pneumonitis, respiratory (smoking-related) bronchiolitis, and endobronchial spread of infection.

Tree-in-bud opacities appear as tiny centrilobular branching structures on CT, most often in the lung periphery, which resemble budding trees ( Figure 18-4 ). These are due to filling of the distal bronchioles and involvement of the adjacent alveoli, most often caused by infectious bronchiolitis, bronchitis, and aspiration.

12

True or false: Consolidation and ground glass opacities indicate pneumonia.

False. While consolidation and ground glass opacities usually indicate an acute reversible process, they can be seen in a wide variety of conditions, including some malignancies.

13

True or false: Consolidation and ground glass opacity can be seen in acute and chronic conditions.

True. Although these abnormalities are more commonly associated with acute illnesses, chronic inflammatory and neoplastic disorders can also present as airspace disease.

14

What are some causes of acute airspace disease?

Blood (such as from trauma or vasculitis), pus (from pneumonia or aspiration), or water (due to cardiogenic or noncardiogenic pulmonary edema) filling the alveoli.

15

What are some causes of chronic airspace disease?

Malignancy (lymphoma, bronchoalveolar subtype lung cancer) ( Figure 18-5 ), organizing pneumonia, mycobacterial and fungal infections, chronic eosinophilic pneumonia, and PAP.

16

What are some causes of diffuse airspace disease?

Cardiogenic and noncardiogenic pulmonary edema, adult respiratory distress syndrome (ARDS), Pneumocystis jiroveci (formerly carinii ) pneumonia (PCP), inhalational lung injury, pulmonary hemorrhage, and PAP.

17

What are some causes of multifocal or patchy airspace disease?

Pneumonia, aspiration, pulmonary contusion, and pulmonary hemorrhage when acute. Organizing pneumonia, mycobacterial infection, fungal infection, chronic eosinophilic pneumonia, PAP, vasculitis, and malignancy (lymphoma, lung cancer, and pulmonary metastasis) when chronic.

18

What are some causes of focal airspace disease?

Pneumonia, aspiration, pulmonary contusion, and pulmonary infarction when acute. Malignancy (lymphoma, lung cancer, and pulmonary metastasis), organizing pneumonia, and lipoid pneumonia when chronic.

19

What are some causes of peripheral airspace disease?

Pneumonia, pulmonary infarction, and pulmonary contusion when acute. Organizing pneumonia, chronic eosinophilic pneumonia, and vasculitis when chronic.

20

What is atelectasis?

Atelectasis is reduced inflation of part or all of the lung and may be subsegmental, segmental, lobar, or whole lung in extent. Whereas subsegmental atelectasis is usually not clinically significant (despite being very common, particularly in the inpatient setting and after surgery), larger amounts of atelectasis can result in significantly decreased pulmonary gas exchange.

21

What types of atelectasis may occur?

Atelectasis can be passive, compressive, resorptive, cicatricial, or discoid.

Passive atelectasis occurs when the lung is allowed to retract (e.g., due to presence of a pneumothorax).

Compressive atelectasis results from extrinsic compression of the lung by a space occupying process (e.g., a bulla or lung mass).

Resorptive (or obstructive) atelectasis occurs due to airway obstruction (e.g., from mucus plugging or endobronchial tumor) that results in peripheral alveolar gas resorption.

Cicatricial atelectasis results from lung fibrosis (e.g., from prior mycobacterial infection or radiation therapy).

Discoid (or platelike) atelectasis is a form of adhesive atelectasis due to hypoventilation, and it has a bandlike or linear configuration, most often seen in the lung bases.

22

What are the imaging features of atelectasis?

Atelectasis typically appears as a well-defined homogeneous area of lung opacification, sometimes with air bronchograms, that is often confined to a pulmonary segment or lobe, although it can range from a minimal linear opacity to complete lung opacification ( Figure 18-6 ). This is frequently associated with decreased lung volume as well as abnormal shift of pleural fissures, bronchi, and pulmonary vessels toward the atelectatic lung parenchyma. As atelectasis represents normal but nonaerated lung tissue, it will enhance uniformly on CT or magnetic resonance imaging (MRI) after intravenous contrast administration. This is in contrast to other airspace opacities such as by pneumonia and tumors which tend to have more heterogenous and variable enhancement. Indirect signs of atelectasis may include ipsilateral diaphragmatic or cardiomediastinal shift toward the atelectatic lung parenchyma, ipsilateral rib space narrowing, and compensatory hyperinflation of unaffected portions of the lungs.

23

What is rounded atelectasis, and what is the “comet tail” sign?

Rounded atelectasis is a form of chronic atelectasis that occurs next to sites of pleural fibrosis and is the result of retraction of visceral pleural scar. Over time, this results in infolding of the lung, which eventually develops a rounded masslike appearance. To confidently establish the diagnosis of rounded atelectasis, one must see a rounded subpleural area of opacity with an adjacent pleural thickening, and curvilinear bronchovascular structures extending into the margin of the opacity (the latter of which is known as the “comet tail” sign) ( Figure 18-7 ).

24

What is the “Luftsichel” sign?

The “Luftsichel” (air sickle) sign represents a small curvilinear lucency that parallels the left upper mediastinum on a frontal chest radiograph. This occurs in the setting of left upper lobe atelectasis due to interposition of the superior segment of the left lower lobe between the medially displaced and collapsed left upper lobe and aortic arch.